1. Field of the Invention
This invention relates generally to an improved zener or reference diode and fabrication method therefor for integrated circuits and, more specifically, to an integrated reference diode having a subsurface breakdown over a larger range of working currents and reduced sensitivity to both intended and random process variations.
2. Brief Description of the Prior Art
In the past, integrated circuit diodes having controlled breakdown voltages have been fabricated with the processes used for forming and isolating transistor elements required by the circuit. Examples of such diodes may be found in the Dobksin U.S. Pat. 3,886,001 and the Tsang U.S. Pat. No. 4,213,806. Such diodes suffered from two significant limitations. First, since the diodes were formed by substantially the same process steps used for the transistor devices in the integrated circuit, a change in the process to modify the transistor characteristics could undesirably impact the diode characteristics. Second, process variations within normal limits could result in unacceptable diode characteristics, such as insufficient dynamic range, excess noise, poor control over impedance, and the like.
Thus, a need existed to provide a diode structure and process which both resulted in better diode characteristics and reduced the sensitivity to intentional and unintentional process variations. Specifically, a need existed to provide a diode structure for operation in the reverse or breakdown mode whereby bulk breakdown characteristics can be obtained over a reasonable range of variation of process parameters.